Effects of processing on microstructure and properties of SLS Nylon 12.pdf

Materials Science and Engineering A 435436 (2006) 172180 Effects of processing on microstructure and properties of SLS Nylon 12 H. Zarringhalama, N. Hopkinsona, N.F. Kampermanb, J.J. de Vliegerb aRapid Manufacturing Research Group, School of Mechanical and Manufacturing Engineering, Wolfson Building, Loughborough University, Loughborough LE11 3TU, United Kingdom bTNO Science and Industry, De Rondom 1, P.O. Box 6235, 5600 HE Eindhoven, The Netherlands Received 13 February 2006; received in revised form 9 June 2006; accepted 21 July 2006 Abstract There currently exists the requirement to improve reproducibility and mechanical properties of SLS Nylon parts for rapid manufacturing (RM). In order to achieve this, further fundamental research is needed and this paper addresses this need by investigating effects of potential sources of the lack of reproducibility and reports effects in relation to crystal structure, microstructure, chemical structure (molecular weight) and mechanical properties. Different ? crystal forms were identifi ed and related to the unmolten particle cores and the melted/crystallised regions of the microstructure.Themeltpointofthe?-formvarieddependingonprocessingconditions.Observabledifferenceswerealsopresentwhencomparing the microstructure of the parts. Molecular weight of parts was signifi cantly higher than virgin powder but used powder also showed an increase in molecular weight. This was related to improved elongation at break of parts built from the used powder, consistent with previous studies. Tensile strength showed some increase with machine parameters selected for improved strength but Youngs modulus values were broadly similar. 2006 Elsevier B.V. All rights reserved. Keywords: Selective laser sintering; Nylon 12; Crystal structure; Microstructure; Molecular weight 1. Introduction Rapidmanufacturing(RM)isafamilyoftechnologieswhere products are made in an additive way directly from 3D CAD data, without the need for tooling. The technology was origi- nally known as rapid prototyping (RP) since the properties of parts produced were generally only suitable for prototype parts. However, the technology has evolved to the point where RM is aviablemanufacturingtechniqueinnumerousapplicationareas 1,2. Selectivelasersintering(SLS)isacommonlyusedtechnique in RM 3 and has proved to be suitable for various applications including the creation of bespoke hearing aids 4 and parts for Formula 1 racing cars 5. The aerospace industry in particular has recognised the advantages of SLS for RM where it is cur- rentlythemostwidelyusedtechnique6.Someaircraftalready have numerous SLS production parts as standard 3,7 however the applications are still limited, partly due to the mechanical properties of parts produced. Corresponding author. Tel.: +44 1509 227568; fax: +44 1509 227549. E-mail address: mmhzlboro.ac.uk (H. Zarringhalam). Selective laser sintering produces parts by using a laser to selectively sinter individual layers of a material in a powder form (polymers, metals, ceramics). SLS systems are currently availablecommerciallyfromtwodifferentmanufacturerswhich are3DSystemsoftheUnitedStates(previouslyDTM)andEOS of Germany. Prior to build a CAD model must be created and then processed which includes slicing the CAD model into 0.10.15mm thick 2D layers. After this the data is sent to the machineforpartbuildingwhichisdescribedinTable1.Follow- ing build the part is removed from the unsintered powder and loose particles brushed and/or sprayed off gently using com- pressed air. Semi-crystalline polymers (predominantly Nylon 12 and 11) can be successfully sintered with superior mechanical proper- ties than amorphous polymers 8,9. However, shrinkage during crystallisationhindersproductionofaccurateparts10.Forthis reasonitisessentialthatformaterialstobeprocessedbySLSthe melt temperature be considerably higher than the crystallisation temperature so that crystallisation can be delayed and reduced during the build process 11 to allow new layers to bond to pre- vious layers with a more homogenous microstructure. A high enthalpy of fusion is also preferable to prevent melting of pow- der particles local to the particles targeted by the laser due to conduction of heat. In addition, during laser sintering, a narrow 0921-5093/$ see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.07.084 H. Zarringhalam et al. / Materials Science and Engineering A 435436 (2006) 172180173 Table 1 Part build steps Main part build steps 1. Thin layer of powder deposited across part bed 2. Laser draws cross-section matching corresponding layer in STL fi le, bonding particles and adjacent layers 3. Platform in part-build cylinder moves part downwards a layer 4. Steps 13 repeated until build completed 5. Cooling down of part cake melttemperaturerangeincombinationwithalowmeltviscosity isrequiredtoachievethenecessaryleveloffl uidityveryquickly without inputting excess energy 11. Molecular weight is well known to be a critical characteristic whendescribingpolymersingeneral12andtheweightaverage molecular weight (Mw) directly affects the melt viscosity 10. The melt fl ow procedure is used to characterise melt viscosity by the melt fl ow index (MFI). Low MFI is associated with high melt viscosity and high molecular weight. Gornet et al. 13 and Shi et al. 10 investigated the effects of powder melt viscosity and related lower MFI (and therefore higher melt viscosity and Mw) to improved mechanical properties particularly elongation at break (EaB). It is standard practice in industry to blend virgin powder with used to maintain acceptable part properties which deterioratewithcontinueduseofusedpowder13.Thethermal load during the build process is presumed responsible for the increase in molecular weight and thus increased melt viscosity. Virginpowderisnormallynotusedonitsowninordertoreduce associatedcosts.ForRMconsistencyinmechanicalpropertiesis particularlyimportantandsoitisimportanttotackletheexisting potential for lack of reproducibility of properties. Nylon 12, as with other aliphatic Nylons, has been widely shown to form stable crystal structures of either the ?- or ?- form 14,15. For Nylon 12 the ?-form, which is more stable, always melts at a higher temperature than the ?-form 16. It has been demonstrated that under the majority of processing condition the liquid phase will crystallise to the ?-form, which is therefore the most commonly found crystal form in Nylon 12. However, certain conditions do result in the ?-form. These have been reported as solution casting (at atmospheric or reduced pressure, below 90C) 16, drawing just below the melt point and high pressure crystallisation 21. It has been demonstrated thattheproportionsof?-and?-formcanbealtered(from0%to 100% for each) by controlling the thermal history 16,17. Dif- ferent variations of the basic ?- and ?-crystal forms have been created depending on processing conditions and these can be transformed from one to another by numerous methods depend- ing from which and to which crystal form the material will be transformed 1821. Studies reporting melting points for the ?-form were reviewedbyAharoni15andvaluesrangedfrom172to185C with most data points close to 179C. The melt point of the less common ?-form has been reported as ca. 173C 16. SLS Nylon is obtained by dissolving polyamide-12 in ethanol under pressure at elevated temperatures followed by slow crystalliza- tiontherebyformingcrystalswitharelativelyhighmeltingpoint of ca. 190C and a relatively high heat of melting 9,11. The crystal structure of SLS Nylon powder or parts has not been reported on however the slow cooling rate during production would likely result in the virgin powder being comprised of rel- atively large crystals. The production process for SLS Nylon 12 does not exactly meet the requirements for formation of ? crys- tal form and the high melt temperatures measured indicate the ?-form. Published information relating to alternative methods for manufacture of SLS powder describes how standard Nylon 12 can be modifi ed to increase the melt temperature with insignifi - cantincreasetothecrystallisationtemperature22.Theprocess described involves heating powder or granules of the material in steamforextendedtimeduration(upto100h).Itisclaimedthat this increases the melt temperature by allowing the molecular chainstoberearranged.Itisknownthatevennumberednylons suchasNylon12canonlyachievefullinter-molecularhydrogen bonding and thus crystallisation by the molecular chains align- inginananti-parallelarrangement14anditisthoughtthatthis is how the chains are rearranged. Youngs modulus and tensile strength of selective laser sin- teredNylon12arecomparablewithvaluesforstandardinjection mouldedsamples23.HoweverEaB(whichindicatesductility) isatleastanorderofmagnitudelower.Gornetetal.13demon- strated how mechanical properties can vary between different machines and even for the same machine in different builds. Salehetal.24andGibsonandDongping25showedconsider- ablevariabilityfordifferentpartorientationandpartsbuiltindif- ferent locations in the build volume within the same build. Ton- towi and Childs 26 investigated the effect of powder bed tem- perature (ambient build powder surface temperature) on density and showed that small variations in temperature have a marked effect on part density which can affect mechanical properties. TheSLSmachinemanufacturerssupplypowderandtheirkey published mechanical (and other) properties, for their respec- tive SLS Nylon 12 powders, are summarised in Table 2. Since the machine parameters used to generate the data are not given by both manufacturers and because the data is generated using different standards, this cannot be compared directly. It should therefore only be used to roughly gauge the potential of SLS Nylon 12. Where manufactures quoted a range of values, the average has been given. From the application of SLS as a production process, the requirement to improve reproducibility and mechanical proper- ties of SLS Nylon parts for RM exists. In order to achieve this, furtherfundamentalknowledgeoftheunderlyingmechanismsis needed. This paper addresses this need by investigating effects Table 2 Manufacturers published materials properties for SLS Nylon 12 27,28 3D SystemsEOS Tensile strength (MPa)4445 Tensile modulus (MPa)16001700 Tensile elongation at break (%)920 Part melting point (C)184184 Particle size, average (?m)5858 Particle size, range 90% (?m)2592Not available Part moisture absorption, 23C (%)0.410.52 174H. Zarringhalam et al. / Materials Science and Engineering A 435436 (2006) 172180 of build procedure, parameters and powder blend and reports effects in relation to crystal structure, microstructure, chemical structure (molecular weight) and mechanical properties. 2. Methodology Different specimens of SLS Nylon 12 powder and parts were analysed. Powders with different thermal histories were analysed and parts were produced from these. These are sum- marised in Table 3 and explained further in Section 2.1. Parts and powder were obtained from TNO Science and Industry and the Rapid Manufacturing Research Group, Loughborough Uni- versity. These are referred to as TNO and Lboro, respectively. Different machines were used which were already confi gured with contrasting parameters. This was intended to provide a widerrangeofdata.EachmachinewasusedwithSLSNylon12 powder supplied by the respective machine manufactures. The differences are explained further in Section 2.2. The powder specimens were analysed for molecular weight distribution by gel permeation chromatography (GPC), thermal properties and crystal structure by differential scan- ning calorimetry (DSC), and microstructure by optical micro- scopy. Part specimens were also tensile tested for mechanical properties. 2.1. Preparation of powder Powder was prepared in different ways prior to building, as follows: 1. Virgin: Powder that has never been processed in a laser sin- tering machine. 2. Used: Part cake and overfl ow powder that has been through a complete SLS run. 3. Refreshed: A mix of 67% used and 33% virgin powder. 2.2. Part production Standardtensiletestspecimens(asshowninFig.1)werepro- duced according to ISO 527-2 1A. The parameters of the TNO machine were set for optimized mechanical properties while those for the Lboro machine were confi gured for optimized accuracy which was anticipated to give reduced mechanical properties.Bothmachinescanbeconfi guredforeitherimproved mechanical properties or improved accuracy and so the choice Table 3 Specimen history Source Specifi csSpecimens PowderParts from TNO EOS PA2200 powder, EOS P380 machine (tuned for mechanical properties) VirginVirgin Used RefreshedRefreshed Lboro 3D Systems Duraform PA powder, 3D Systems Vanguard machine (tuned for accuracy) VirginVirgin UsedUsed RefreshedRefreshed Fig. 1. Build setup. of confi guration for this research was simply based upon the existing confi gurations already in use by the different operators. ThepowderbedtemperatureoftheLboromachinewasadjusted for each type of powder while the powder bed temperature of the TNO machine was kept constant for all powder types. Dif- ferencesbetweenthetwodifferentmachinesandtheirparticular build setups are listed in Table 4. In order to ensure thermally stable conditions a base layer of two centimetres and a top layer of one centimetre were used for each run. Parts were built fl at, laid parallel to the machine x-axis, at fi xed places in the middle of the build area as shown in Fig. 1. 2.3. Molecular weight The molecular weight was determined by GPC by drying 25mg samples at 120C for 1h. The samples were then dis- solved at room temperature in 5ml chloroform (stabilized with ambilene) and 0.5ml trifl uor acetic acid anhydride (TFA). After 2h dissolving the solution was diluted with 20ml chloroform. This solution was fi ltered without heating and analysed using two WatersTMStyragelHT6E, 7.8mm300mm columns in combination with a differential refractive index detector (WatersTM410) and a tuneable absorbance detector (WatersTM 486) at wavelength 264nm. Polystyrene was used as the refer- ence material. Table 4 Machine differences TNOLboro Parameter optimisationMechanical propertiesAccuracy MachineEOS P3803D Systems Vanguard MaterialPA2200 Nylon 12Duraform PA Nylon 12 Layer thickness0.15mm0.1mm Powder deposition mechanism HopperRoller Feed powderCold (50C)Heated (+100C) Build platformHeatedUnheated Laser power (fi ll)45.7W (90%, 50W)11W Laser power (outline)10.9W (20%, 50W)5W Scan speed4000mm/s5000mm/s Scan spacing0.3mm0.15mm H. Zarringhalam et al. / Materials Science and Engineering A 435436 (2006) 172180175 2.4. Thermal properties Differential scanning calorimetry (DSC) was used to deter- mine thermal properties in order to study melting and crystal- lizationbehaviourandtoidentifycrystalforms.Samplesofmass ca. 10mg were analysed in a Thermal Analysis DSC machine under nitrogen fl ow. Samples were heated from room temper- ature at 10C/min to 200C and then immediately cooled at 10C/min. 2.5. Microstructure The microstructure of the processed materials was analyzed via optical microscopy. Samples of 35?m thickness were cut at room temperature by microtome. Images were taken in bright fi eld polarized illumination. 2.6. Mechanical properties Tensiledatawasrecordedusingaclip-onextensometer.Each tensile test was executed with fi ve individual specimens and results for tensile elongation at break, ultimate tensile strength and Youngs modulus were recorded. 3. Results and discussion 3.1. Powder 3.1.1. Virgin powder: comparison of Lboro and TNO powder by thermal analysis and molecular weight analysis Fig. 2 shows DSC results for Lboro and TNO virgin pow- der. Both show relatively sharp single endotherms with melting peaks at 189C for TNO virgin powder and 188C for Lboro virgin powder. These correspond with known values for virgin powder 29,11 however they are at least 3C higher than the previously reported values for crystal forms of non-SLS pro- cessed Nylon 12 (172185C). Fig. 2. DSC results for Lboro and TNO virgin powder. Fig. 3. GPC results for Lboro and TNO virgin powder. As mentioned previously, the conditions in the production of SLSNylon12powderdifferfromthoserequiredtoform?-form crystal structure and this suggests that the endotherms obtained from the virgin powder relate to the common ?-form. On the basis that prolonged molecular mobility increases the melt tem- peratureitcanthereforebeinferredthattheendothermsrelateto the?-formwithfarlargerandmoreperfectcrystalstructure(due toincreasedhydrogenbonding)thanpreviouslyanalysedNylon 12 specimens. Additionally, Ishikawa and Nagai 16 demon- stratedthatasingleendotherminaDSCtestonNylon12isonly characteristic of a sample containing 100% ? crystal form since samples consisting of 100% ? crystal form invariably develop a second melt peak during the DSC run due to recrystallisation of a portion of the specimen to the ? crystal form. The test param- eters were the same as those used in the present investigation supporting the view that the crystal structure present in virgin SLS Nylon 12 material is of the ? crystal form. Fig. 3 shows GPC results for Lboro and TNO virgin powder. The relative weight average molecular weight is 90,000g/mol for Lboro virgin powder and 70,000g/mol for TNO virgin pow- der. The polydispersity index is 2.26 for Lboro virgin powder and 2.34 for TNO virgin powder. The small peaks on the posi- tivetailofeachcurvearepossiblycharacteristicofanti-oxidants, whichareknowntobeblendedwiththevirginNylon12powder from EOS 28 and therefore presumed to be the same with 3D Systemssuppliedpowder.HighermolecularweightoftheLboro powdercouldtheoreticallyhinderprocessingandthusproperties 30 however the difference is too small to be signifi cant. 3.1.2. Sub-m